Field
Embodiments generally relate to the art of inertial confinement fusion, and more particularly to driver lasers for inertial fusion energy and high energy density science applications.
Related Art
Inertial confinement fusion (ICF) is the science and technology of achieving controlled thermonuclear fusion wherein matter is compressed and heated to extreme conditions, a state often described by high energy density physics (HEDP), such that nuclear reactions occur. [Nuckolls 1972, Nuckolls 1982]
The primary embodiment of inertial confinement fusion utilizes a very large laser system to irradiate a small target containing a small amount of fuel comprised of light nuclei, thereby compressing and heating the fuel to warm density matter conditions. The fuel remains in this state of high energy density for a very short time, just long enough for significant nuclear energy to be released. A related field is inertial fusion energy (IFE), which is the science and technology of using inertial confinement fusion in a commercial power plant to produce electricity for commercial use through the national power grid, and other applications. [Campbell 1999a, Campbell 1999b, Hogan 1991, Campbell 1999c, Moses 2009, Nardella 2008, Storm 1991, Dunne 2012, Obenschain 2011]
The US government has supported studies of ICF and IFE for many years. For ICF the goal is to support the nuclear weapons programs at the National Laboratories, and as such the programs have been administered by the National Nuclear Security Administration (NNSA). [Campbell 1999c, Paisner 1994, Campbell 1999b, Nardella 2008] The NNSA's mandate is nuclear security—it has no mandate in IFE. The NNSA programs are therefore specifically limited to nuclear security. As such, a goal of the NNSA programs has been to develop the capability to create significant neutron fluences, by the most rapid, cost-effective, and reliable path. This path is substantively different from the development path for IFE. Thus, the NNSA programs have not had a focused and extensive effort on the development of the possible optimal technological approaches to ICF/IFE in civilian and commercial applications. The prior art is therefore interesting as background and shows some features of the architecture that can be utilized for IFE, but is silent on some of the essential technologies for IFE/ICF.
The nuclear fusion reaction that has been the primary focus of ICF under U.S. government funding is the nuclear reaction between deuterium (D) and tritium (T) nuclei, which are isotopes of hydrogen. The DT reaction can efficiently produce a helium nucleus (a) and a neutron (n), releasing significant energy. By collecting the products of the reaction, the released energy can be collected and transformed into heat and electrical power. There are several national security applications of ICF including supporting the scientific base for nuclear weapons (Stockpile Stewardship Program), producing materials such as tritium for the nuclear deterrent, and destroying (transmuting) spent nuclear fuel from fission reactors. Both ICF and IFE include the study of other nuclear reactions, and so are not restricted to the DT reaction. Fusion can also be used in conjunction with fission to produce energy in “hybrid systems.”
In laser-driven inertial confinement fusion (ICF) [Nuckolls 1972, Nuckolls 1982], a large laser irradiates a small target containing an approximately spherical capsule containing fuel comprised of carefully selected light nuclei. This approach is called “direct drive.” [McCrory 2011]. In an alternative approach called “indirect drive,” the laser energy is first converted into x-rays in an enclosure (called a hohlraum) containing the fusion capsule [Lindl 1995]. The laser irradiation (or x-rays for indirect drive) ablates the outer surface of the capsule. The ablated material has a high momentum, and acts on the capsule in a manner similar to a rocket engine, forcing the capsule surface inward. As the capsule implodes in response to the ablation forces, the fuel contained by the capsule is compressed and heated. After the laser pulse has ended, the capsule continues to implode, coasting inward to smaller size, but slowing down. At some point in time, the capsule stagnates briefly. Thereafter, the capsule disassembles (explodes) under its own pressure and the pressure of the fuel it contains after significant thermonuclear reactions have taken place. At stagnation, the fuel is ideally in a state of high energy density, which is defined as having a temperature and density such that nuclear reactions take place. During the very brief moments of stagnation (about 10−10 seconds), the fuel pressure reaches values of 200 billion atmospheres (higher than that found in the center of many stars), and energy is released by the nuclear reactions in the form of kinetic energy of the particles produced or created by the nuclear reaction. Some of the released energy is captured in the fuel, heating the fuel further, and some of the released energy escapes and is captured for subsequent use. In addition to this standard approach of compression and heating that occurs through the action of a single driver laser, other approaches have been investigated for igniting the fuel after it has been compressed. These include shock ignition. [Theobald 2008, Theobald 2009, Perkins 2009] and fast ignition [Tabak 1994, Deutsch 1998, Campbell 2006], which are both alternative approaches to initiating the burn of DT in high energy density matter.
The time during which nuclear reactions occur, or the time during which high energy density matter conditions are maintained, is essentially the time elapsed while the fuel and capsule stagnate, before they accelerate outwards under the action of their own pressure. The fuel and capsule resist this outward acceleration simply by their own inertia. Thus, it is the inertia of the fuel and capsule that tends to maintain the high energy density condition achieved at stagnation. Therefore, nuclear energy is produced only during the time the fuel is inertially confined at stagnation. Ideally, the stagnation time is long enough to permit many nuclear reactions to take place and significant usable energy to be released. Thus, the art is termed “inertial confinement fusion.”
The art of inertial confinement fusion (ICF) divides generally into two areas, targets (i.e., fuel and capsule) and drivers, with driver lasers being the most common. Both areas involve complex science and technology factors, making ICF technically challenging. Experiments to date on ICF have indicated the general features of the driver laser, and the structure of the targets, but significant uncertainties remain to be resolved in both areas.
For inertial fusion energy (IFE), an ICF core is embedded in a power plant. To produce useful amounts of electrical power for the national grid (typically 100 MW to 1 GW), targets must release about 50-100 times the laser energy used to drive the implosion, and they must be irradiated by the laser several times a second (˜5 to 15 Hz, depending on target energy yield). The balance of the power plant deals with the technology of capturing the energy released and converting it into electricity for the national grid or other uses, the technology of target fabrication, and laser operation. The whole enterprise is constrained by the cost of electrical power (USD/MW-hr) supplied to the national grid. Power plants currently sell electricity to power distributors at a rate between 100 and 150 USD/MW-hr. In IFE, there are many additional factors to consider beyond the science and technology of ICF. For the laser, these include the need to fire the laser several times a second, the need to limit the down-time of the laser system for maintenance, the need to preserve the quality of the optical pulses used to irradiate the target, its wallplug efficiency and its capital cost.
As one might imagine, there are many complex scientific and technological aspects to ICF and IFE. Some of these aspects have been identified, but significant experimentation and technological developments will be required before a practical IFE power plant can be designed and operated.
A laser system for driving an inertial fusion target must meet a stringent set of requirements [Bayramian 2010, Bayramian 2011, Orth 1996, Caird 2009]. The requirements include the following: a total laser energy greater than about 1000 kJ (significant research may be able to reduce this to ˜500 kJ); a small focal spot typically around 500 microns in size; a wavelength in the visible or ultraviolet region typically between 550 nm and 250 nm; a pulse length of a few tens of nanoseconds; a spatial profile of the total intensity such that on short time scales of about several picoseconds, the intensity profile in the focus is uniform; a bandwidth adequate to suppress laser-plasma and hydrodynamic instabilities, typically greater than a few THz; and a complex temporal pulse shape that compresses the DT fuel without excessive heating early on, typically beginning with a short spike lasting less than 1 nanosecond followed by a smooth rise over many nanoseconds to a peak, followed by an approximately constant power for a few nanoseconds. For energy applications, the laser system must include a means of measuring the position and orientation of a target moving at a high velocity of approximately 100 m/s, pointing the laser system at the target, and delivering the correct laser pulse format to the target. In addition, the laser system must typically do this several times a second. These requirements are challenging to meet in one and the same laser system.
The configuration of the driver laser is constrained by available laser technology. Numerous research studies over the past 40 years have shown that target physics requires an illuminating wavelength in the ultraviolet (250-350 nm) or perhaps visible (500 nm) ranges. These wavelengths are either produced directly as in the output of KrF lasers [Sethian 2002] or by non-linear frequency up-conversion of the fundamental 1053 nm wavelength of Nd:glass lasers to 355 nm. [Paisner 1994, McCrory 2012] The wavelength is constrained by the damage threshold of currently available optics. In the ultraviolet range, today the damage threshold of the optics at the exit aperture is typically 1-2 GW/cm2, or equivalently 3-5 J/cm2. To provide a megajoule of ultraviolet energy, given current damage thresholds, the total area of the exit aperture of all the lasers is approximately 100 square meters. There is some advantage to using visible or infrared wavelength, because the damage threshold is significantly higher than in the ultraviolet at 355 nm. There are many other requirements on the laser system for inertial fusion energy production, but here we are concerned mostly with the configuration of the laser(s) themselves. Given the total energy required, the damage threshold for optics, physics and manufacturing constraints on laser aperture size, fusion capable ICF and proposed prior IFE laser systems typically have one to several hundred identically configured laser beams with nominal aperture size of 20-40 cm.
The driver laser for the most common approaches to inertial fusion typically delivers just one pulse to the target that compresses the DT fuel and causes the fuel to ignite and burn. In several advanced approaches such as the fast igniter [Tabak 1994, Deutsch 1998, Campbell 2006], as mentioned above, two laser pulses are envisioned with different functions, one to compress the DT fuel and another to ignite the fuel. The fuel igniter laser requirements are generally quite different from the fuel compressor laser. Though still uncertain, the igniter requirements include an energy around 100-200 kJ, a pulse length approximately 10 picoseconds, and a focal spot size of tens of microns. In other advanced applications, such as shock ignition [Theobald 2008, Theobald 2009, Perkins 2009], two laser pulses or a single appropriately shaped laser pulse may also be used, one to create a low velocity compression and the second to launch a shock which ignites the fuel.
One of the key requirements for ICF is that the target should maintain its spherical shape while being compressed [Lindl 1995, McCrory 2011, Obenschain 2011]. In direct drive fusion, the laser beams impinge directly on the target itself, and therefore the spatial profile of the laser drive should be highly uniform. Drive nonuniformity causes the shape of the target to deform during the implosion by two distinct mechanisms. The first mechanism is simply that if the acceleration of the shell is not uniform, the shape of the shell changes as it imploded and so either at stagnation the capsule will not be spherical or the stagnation of the capsule will not be simultaneous around the shell. Then the density and temperature of the fuel may not reach ignition values. The second mechanism is that shell perturbations are amplified by the Rayleigh-Taylor and Richtmeyer-Meshkov hydrodynamic instabilities causing cold matter from the shell to penetrate the hot fuel at stagnation. This cold matter can also cause the target to fail to reach ignition. By and large, effects such as hydrodynamic smoothing tend to make the acceleration uniformity requirement more significant at longer spatial scales, and the Rayleigh-Taylor and Richtmeyer-Meshkov hydrodynamic instabilities more significant at shorter spatial scales. In general, the overall uniformity requirement is that the rms nonuniformity should be less than about 0.25% to 1% when integrated over the e-folding time for the instabilities.
Another requirement for ICF is that the energy of the laser should couple relatively efficiently to the implosion. Laser ablation creates a plasma surrounding the target comprised of the ablated materials. The plasma surrounding the target can act as a medium in which the laser can drive laser plasma instabilities (LPI) such as filamentation, stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and the 2ωpe instability. LPI scatters the laser drive and in so doing reduces the energy that is coupled to the implosion and redirects the laser light energy so that it is non-uniform. For example, stimulated Brillouin scattering causes energy to be transferred from one beam to another and changes the sphericity of the drive. Also, some LPI processes, such as SRS and the 2ωpe instability, can produce energetic electrons that can “preheat the fuel” and make compression more energy intensive. The requirement on the laser drive to avoid LPI is essentially that the laser bandwidth should be at least about 1-2% of the laser frequency.
Typically, some type of beam conditioning is required to ensure that the uniformity and bandwidth requirements are met. The laser technology involved is termed “beam smoothing”. There are two existing approaches to beam smoothing: Induced Spatial Incoherence, implemented on Nike [Lehmberg 1998]; and smoothing by spectral dispersion [Skupsky 1989], implemented on NIF, LMJ and Omega, as described below.
As mentioned above, there are several options for the driver laser and the target design. The two primary target options are indirect drive and direct drive. In addition, a third approach is a variant of the direct drive approach, known as polar direct drive. The two primary laser options in the prior art are the NIF-style Nd:glass laser[Paisner 1994] and the KrF laser[Sethian 2002, Obenschain 2011]. A third but impractical laser option utilizes a fiber approach[Labaune 2008]. Because the highest energy gain (fusion yield/driver energy) is believed to be provided with direct drive target designs, direct drive is today the most interesting for IFE. However, the NNSA programs, which are motivated by national security missions, have invested heavily in NIF-style Nd:glass lasers and indirect drive targets. High gain indirect drive concepts have also been proposed that may satisfy the requirements for IFE. However, the current suite of existing NNSA laser facilities is not optimally configured for exploring and demonstrating ignition in direct drive targets. In fact, given its national security mission and focus, a laser concept that meets all the requirements for IFE has not been developed in the NNSA program.
At the same time, the reactor technology (the technology of capturing, converting and using the energy released by the targets) has been funded by a different agency of the US government. The so-called HAPL (High Average Power Laser) program at the Naval Research Laboratory in Washington, D.C., LLNL and other participants, has been funded by Congress to develop KrF and diode pumped solid state (DPSSL) laser technologies and to study reactor technology and other issues associated with IFE such as target production and injection. A great deal of progress has been made on both KrF laser technology [Sethian 2002], on advanced solid state laser approaches[Bayramian 2007], and on some elements of the reactor. However, these studies have been directed to understanding the balance of elements in an IFE power plant and the design of the reactor which captures the energy released from the targets. They have not been directed to understanding how to configure the laser driver so that the full set of target requirements can be met simultaneously.
Under the NNSA program, studies of ICF have been carried out at the Lawrence Livermore National Laboratory (LLNL) [Haynam 2007][Paisner 1994][Campbell 1999], the Laboratory for Laser Energetics (LLE) at the University of Rochester, N.Y., [LLE 2012] and the Naval research Laboratory in Washington D.C. [Sethian'2002]. The earliest studies date from the early 1970's. The lasers used at LLNL and LLE are generally referred to as NIF-style lasers, after the National Ignition Facility at LLNL.
The NIF-style laser [Haynam 2007] is a flash-lamp pumped Nd:glass laser. Several generations of glass lasers have been built at LLNL, each one significantly larger than the previous laser. The configuration of this laser is a master-oscillator/power amplifier, in which a small laser beam with the desired pulse shape and spatial mode properties is first generated in the master-oscillator and then amplified in a series of power amplifiers and transport optics before being focused onto a target[Paisner 1994]. The more recent Nd:glass lasers have included a frequency conversion device that converts the wavelength of the laser beam from 1064 nm in the infrared to 355 nm in the ultraviolet, before the wavelength-converted beam is focused onto the target, as well as other “beam conditioning systems such as phase plates to control the focal spot profile [Paisner 1994, Haynam 2007]. This laser approach is mature (for example, NIF is the sixth laser build by LLNL since the early 1970's) and so has minimal risk as a choice for a driver laser for ICF. While low risk may be an attractive feature for nuclear security programs, when NIF is measured against the requirements for IFE, several short-comings are evident. For example, its efficiency (optical energy out/electrical energy in) is less than 1%, whereas the requirement is around 10%. It has a very narrow bandwidth, about 0.25 THz, whereas the requirement to suppress LPI is believed to be around 1-2% of the laser frequency, or perhaps greater. The smoothing technique used by NIF at LLNL and Omega[LLE 2012] is smoothing by spectral dispersion (SSD)[Skupsky 1989, Skupsky 1993], which is essentially a means of causing the laser beam to shimmer at a high rate at the target. The smoothness of the LLNL laser pulses at the target is significantly higher than 20%, and the smoothness of the LLE laser pulses is about 10%, to be compared with the requirement which is typically 0.25%. It is difficult if not completely impractical to configure a NIF-style laser to deliver more than about 1 pulse per second, without significant advances in laser amplifier technology, whereas the requirement for IFE is about 5-15 pulses per second. Achieving a high repetition rate a NIF-style laser is challenging because of the large beam aperture. Also the wallplug efficiency of a NIF-style laser is ˜1% today. Again with significant advances in laser architecture and amplifier design this can be improved, but it is unlikely to meet the IFE requirement.
The NIF-style laser configurations used by LLNL have included several separate beam lines, all nominally identical and delivering the same pulses to the target. The latest system at LLNL, the National Ignition Facility [Paisner 1994][Campbell 1999a][Haynam'2007] uses 192 beams in 48 clusters delivering pulses to a target chamber about 5 meters in radius, containing a target placed at its center. The final focusing lenses are about 7 meters from the target. The total energy per pulse delivered by NIF is up to 1.8 MJ at 355 nm, delivered in a pulse length of approximately 25 ns. The latest system at LLE is Omega [McCrory 2012] which uses 60 beams and delivers about 30 kJ of energy in the ultraviolet in about 1 nanosecond.
Variants of the NIF-style laser adapted to some of the special requirements of commercial energy production have been studied [Caird 2009, Storm 1991, Campbell 1999a, Hogan 1991], and some initial experiments have been carried out to test them [Bayramian 2007].
The laser used at NRL is Nike, a Krypton Fluoride (KrF) laser. [Sethian 2002] The KrF laser operates in the ultraviolet at 248 nm. It also is configured as a master-oscillator power amplifier. The KrF laser has the best smoothness achieved to date, which is less than 1%. The KrF laser uses the smoothing technique of induced spatial incoherence [Lehmberg 1993, Lehmberg 1998, Lehmberg 2000, Lehmberg 2005]. The Nike laser bandwidth is less than 5 THz, and its efficiency can potentially be as high as 7%. The optical configuration of the KrF laser involves passing a few short pulse beams in sequence through an amplifier and then passing each beam through an optical delay so the beams arrive at the target simultaneously. The KrF laser has some attractive features for IFE[Obenschain 2011], compared to the NIF-style laser, a flexible KrF design that meets all the requirements has proved elusive.
A regular feature of the prior driver lasers is that their configuration is typically master-oscillator/power-amplifer (MOPA), where a primary oscillator provides a single small seed pulse that is optically divided using beam-splitters into many seeds. All seed pulses are therefore coherent with each other. Individual seed pulses may be shaped and modulated in slightly different ways. Each seed pulse passes through a separate amplifier or a system of amplifiers, one for each seed, where the seed pulse is amplified and subsequently focused onto the target. Thus the pulses from all the beams focused onto the target are coherently related. For the NIF-style laser, the primary seed is highly spatially coherent. The modulation is used in conjunction with diffraction gratings according to the SSD methodology. The optical phases of the beams across each beam aperture are therefore highly coherent. Even after passing through a phase plate, a high degree of spatial coherence remains. Thus every time the laser fires, the same phase relationships exist between the beams impinging on the target which compromises the smoothness of the drive at the target. Indeed, it is challenging for a Nd:glass laser configured as in the prior art to deliver a laser drive at the target with the required smoothness for direct drive target.
For the KrF laser system, the primary seed is multimode, and after being split, it is amplified and imaged onto to the target. The asymptotic smoothness of the drive at the target is limited by the spatial intensity profile at the target associated with each spatial mode, and the rate of smoothing is controlled by the bandwidth. While the KrF laser has achieved the best asymptotic smoothness in the prior art, the smoothing rate is limited by the (gain-narrowed) bandwidth of the KrF laser amplifiers, which is typically 1-2 THz.
Other lasers for ICF are planned. The Laser MegaJoule in France [Bettinger 1999] was developed in close scientific collaboration with LLNL. It is not yet completed, but as currently planned it will have the same smoothness, bandwidth and mean wavelength as the NIF-style laser at LLNL. It does not represent a substantively different approach to ICF and IFE from NIF. NIF-style lasers are also being considered in Europe (e.g., HiPer) [Dunne 2007] and Japan (Firex) [Azechi 2006], Russia (Unnamed) [Dean 2012] and China (Divine Light 4) [Dean 2012]. All these systems have the same general features as NIF in regard to smoothness, bandwidth, and mean wavelength. Consequently, none of them meet all the requirements for direct drive ICF or IFE. These laser systems all have a few, large aperture (˜40 cm) beams, and face challenges of adequate flexibility in pulse shaping, frequency conversion, bandwidth, beam smoothness, and smoothing rate. The recent proposal for an IFE demonstration known as LIFE (Laser Inertial Fusion Energy) [Bayramiam 2009, Bayramian 2010, Caird 2009, Moses 2009] is an adaptation of the NIF-style laser to IFE, and so it too will have significant challenges to meet all of the IFE requirements.
A different approach has been proposed by an international group of scientists where the laser system is built from a very large number (more than 10,000,000), very small individual single-mode Ytterbium fiber lasers [Labaune 2008]. Each fiber laser output is collected by a lens and focused on the target. To focus on the target, the lens diameter has a minimum size, and this limits the number of lenses. The lens size is such that the entire 4 π solid angle surrounding the target would be significantly filled by approximately 10-20 million lenses. The number of fiber lasers must obviously be significantly less than this. To deliver one megajoule of light, each laser must deliver significantly more than 100 mJ. This is well in excess of the state of the art in single mode fiber laser technology, which is about 10 mJ. Moreover, there are optical engineering challenges such as beam pointing that are extreme in this approach. Even though the fiber approach has been described in the literature related to IFE, it could not be used for IFE without significant invention and development in both large mode area fiber lasers, and precision optical alignment.
All of the laser systems either utilized or conceived in the prior art face significant challenges in meeting simultaneously all the requirements for a successful implosion of a high gain target for ICF or IFE, and all of them have limited flexibility to accommodate different target designs.